A stroke (sometimes called a cerebrovascular accident (CVA)) is the rapidly developing loss of brain function(s) due to disturbance in the blood supply to the brain, caused by a blocked or burst blood vessel. This can be due to ischemia (lack of blood flow) caused by blockage (thrombosis or arterial embolism) or due to a hemorrhage (leakage of blood). As a result, the affected area of the brain is unable to function, leading to inability to move one or more limbs on one side of the body (hemiplegia), inability to understand (receptive aphasia) or formulate (expressive aphasia) speech, or inability to see one side of the visual field (Homonymous hemianopsia).

A stroke is a medical emergency and can cause permanent neurological damage, complications, and death. It is the leading cause of adult disability in the United States and Europe and It is the number two cause of death worldwide. Risk factors for stroke include advanced age (Old age), hypertension (high blood pressure), previous stroke or transient ischemic attack (TIA), diabetes (diabetes mellitus), high cholesterol (hypercholesterolemia), cigarette smoking (tobacco smoking) and atrial fibrillation. High blood pressure is the most important modifiable risk factor of stroke.

A stroke is occasionally treated with thrombolysis ("clot buster"), in a "stroke unit". Secondary prevention may involve antiplatelet drugs (aspirin and often dipyridamole), blood pressure control, statins, and in selected patients with carotid endarterectomy and anticoagulation (Anticoagulant). Treatment to recover lost function is stroke rehabilitation, involving health professions such as speech and language therapy, physical therapy and occupational therapy.